Permanent magnet having an enclosing yoke structure with pole aligning means



' April 8,1969; f Q G ET AL I 3,437,963

I -PERMANENT MAGNET HAVING AN ENCLOSING You.

STRUCTURE WITH POL LIGNINGYMEANS Filed v v June 1967 FI 2 -2 r v v I 28 I l l NVENTORS ROBERT E. GANG l0 RD JOHNSON ORN United States Patent Office 3,437,963 Patented Apr. 8, 1969 3,437,5'63 PERMANENT MAGNET HAVING AN ENCLOSIN G YOKE STRUCTURE WITH POLE ALIGNING MEANS Robert E. Gang, Sunnyvale, and Richard L. Johnson,

Menlo Park, Calif., assignors to Varian Associates,

Palo Alto, Calif., a corporation of California Filed June 12, 1967, Ser. No. 645,392 Int. Cl. H01f 3/00 US. Cl. 335-298 .6 Claims ABSTRACT OF THE DISCLOSURE A high field permanent magnet is disclosed which is suitable for high resolution nuclear resonance. The magnet includes a pair of coaxially disposed permanent magnets enclosed by a magnetically permeable yoke. The enclosing yoke serves as a flux return path between the magnets and as a shield for shielding the gap of the magnet from external magnetic fields.

The magnet yoke is formed of two bowl-shaped halves. Each half of the yoke carries one of the permanent magnets axially thereof. The two bowl-shaped halves of the yoke are joined together at their lips. A pair of mutually perpendicular aligning surfaces are provided on each of the mating lips of the bowl-shaped sections of the yoke. The permanent magnets are axially aligned by shims inserted between and/or screws passing through the aligning surfaces. Fine adjustments in the parallelism of the pole faces defining the magnet gap are made by a set of jack screws at the outer ends of the permanent magnets. The jack screws are screwed through the yoke to make slight tilting adjustments to the permanent magnets.

Description of the prior art Heretofore, permanent magnets have been built for high resolution nuclear resonance spectrometers. However, in these magnets the yoke structure was open on top and bottom for relatively easy access to the pole caps defining the magnetic gap. In such a case, the pole caps were shimmed by inserting shim stock between the magnet and the pole caps to obtain precise parallelism of the gap defining faces and, thus, a relatively homogeneous field. However, the relatively open yoke structure did not provide adequate magnetic shielding for the gap from externally produced magnetic fields.

Others have proposed permanent magnets for nuclear resonance spectrometers employing a cylindrical yoke to enclose and shield the magnetic gap. Such a magnet is described in US. patent applications Ser. Nos. 512,422 and 512,423 filed Dec. 8, 1965, now Patent Nos. 3,325,- 757 and 3,325,758 and assigned to the same assignee as the present invention. However, when the gap is surrounded by the yoke it is no longer possible to mechanically shim the pole caps to remove residual magnetic field gradients which are typically on the order of 5 gauss per inch. These relatively high residual gradients are too large to be cancelled by electric shims if internal heating of the magnet is to be avoided. Such internal heating produces unwanted temperature dependent drifts of the magnetic field intensity.

Sumimary of the present invention The principal object of the present invention is the provision of an improved permanent magnet.

One feature of the present invention is the provision, in a permanent magnet having a pair of coaxial permanent magnets and an enclosing yoke, of separating the yoke into two mating sections with one permanent ma net aflixed to each of the yoke sections and providing aligning surfaces and adjusting means at the mating joint between the two yoke sections for adjusting the alignment of the permanent magnets to improve the homogeneity of the field produced by the magnet.

Another feature of the present invention is the same as the preceding feature wherein the aligning surfaces include a pair of concentric radially spaced axially directed surfaces with means for adjusting the spacing between the radially spaced surfaces to adjust the axial alignment of the permanent magnets.

Another feature of the' present invention is the same as any one or more of the preceding features including the provision of a pair of axially spaced radially directed aligning surfaces with means for adjusting the axial spacing between the surfaces at points around the circumference of the joint to tilt the axis of one permanent magnet relative to the other to reduce certain residual gradients.

Another feature of the present invention is the same as any one or more of the preceding features including the provision of a plurality of off axis jack screws disposed at the end of at least one of the permanent magnets at the junction thereof with the yoke for tilting the axis of one permanent magnet structure relative to the other to cancel certain residual magnetic field gradients in the gap of the magnet.

Other features and advantages of the present invention will become apparent upon a perusal of the following specification taken in connection with the accompanying drawings wherein:

Brief description of the drawings FIG. 1 is a longitudinal view, partly in section and partly broken away, of a magnet apparatus incorporating features of the present invention,

FIG. 2 is an enlarged detail of a portion of the structure of FIG. 1 delineated by 22, and

FIG. 3 is an enlarged detail view of a portion of the structure of FIG. 1 delineated by line 3-3.

Description of the preferred embodiments Referring now to FIG. 1, there is shown a permanent magnet 1 of the present invention. The magnet 1 includes a pair of coaxially aligned permanent magnet structures 2 and 3 enclosed within a coaxial generally egg-shaped magnetic yoke 4, as of soft iron. The magnet structures 2 and 3 are fixedly secured to the ends of the yoke 4 via a pair of axially directed rods 5 which are threaded at their inner ends and screwed into a mounting plate' 6, as of soft iron. The outer ends of the rods 5 are threaded and a nut 7 serves to pull the magnet structures 2 and 3 tightly against the ends of the yoke 4.

The permanent magnet structures 2 and 3 each include a stack of three disk-shaped permanent magnets 8, 9 and 11, as of Alnico V, polarized in aiding magnetic relation to produce a pair of near poles of opposite polarity and a pair of remote poles of opposite polarity. The remote poles of opposite polarity are interconnected by the loW reluctance magnetic yoke structure 4. A pair of mag netically permeable pole caps 12 are mounted on the mounting plates 6 to define the magnetic gap 13 between their mutually opposed flat faces.

A pair of magnetizing coils 14 are coaxially disposed of the permanent magnet structures 2 and 3 and are Wound on cylindrical coil forms 1'5. The coils are initially energized with a series of high current pulses to magnetize the permanent magnets 8, 9 and 11. After the permanent magnets are magnetized, the coils 1'4 may be used to shift the magnetic field in the gap '13.

The yoke structure 4 includes two bowl-shaped sections which are joined together at their lips by a circum- '3 axially spaced surfaces 17 and 18 (see FIGS. 2 and 3), and a pair of axially directed radially spaced surfaces 19 and 21. A hole 22 is provided in the yoke structure 4 for allowing access to the gap 13. The access hole 22 is shown displaced from the Y axis for the sake of explanation.

The axially spaced radial surfaces 17 and 18 (see FIG. 3) are provided to produce an adjustment in the parallelism of the gap defining faces of the pole caps 12 by producing a relative tilting of the axes of the permanent magnet structures 2 and 3. A plurality of screws 23 are screwed into axially directed tapped bores 24 and 25 spaced at 90 intervals about the circumference of the joint 16. Short lengths of thin strips of magnetic shim stock 26, as of for example 0.001" thick steel tape, are positioned between the surfaces 17 and 18 at various locations around the circumference of the joint 16 to tilt the axis of one permanent magnet structure 2 or 3 relative to the other. The surfaces 17 and 18 are initially machined to be flat to within 0.005". The initial X and Y linear gradients, due to non-parallelism of the pole caps 12, are typically about gauss/inch at a field of 14.5 kilogauss. The shims 26 are positioned and adjusted until the residual X and Y field gradients are reduced to on the order of 100 milligauss per inch.

The radially spaced axially directed aligning surfaces 19 and 21 (see FIG. 2) are provided for adjustment of the axial alignment of the permanent magnet structures 2 and 3. More specifically, the two magnet structures should be coaxial to 0.001" in order to reduce second order YZ gradients in the gap 13. A plurality of adjusting screws 28 are screwed into radially directed tapped bores 29 passing through the mating lips of the bowl-shaped halves of the yoke 4. There are four screws 28 at 90 intervals about the circumference of the joint. The screws 28 are adjusted to produce coaxial alignment of the axes of the magnet structures 2 and 3. Once adjusted the screws 28 are tightened to hold the coaxial alignment. As a result, the residual second order YZ gradients are reduced to an acceptable level. In FIG. 1, the position of screw 28 has been shifted 45 for the sake of explanation.

Although the shimmed aligning surfaces 17 and 18 allow a reduction in the X and Y linear gradients to on the order of 100 milligauss per inch, it is desirable to reduce these gradients still further by mechanical adjustments to on the order of milligauss per inch or less. Accordingly (see FIG. 1), the outer ends of the bowlshaped yoke sections are each provided with four axially directed tapped bores 31. The bores 31 are located off the axis of the magnet near the perimeter of the permanent magnet structures 2 and 3 and are disposed at 90 intervals around the perimeter of the magnet structures 2 and 3. The bores 31 terminate at the inner surface of the yoke 4 and do not penetrate the magnet structurs 2 and 3.

Steel plugs 32 are positioned in the ends of the bores 31 to bear against the permanent magnet disks 8. The plugs 32 are freely slideable in the bores 31. Steel set screws 33 are screwed into the bores 31 to bear against the plugs 32 and to transmit adjustable axial forces to the permanent magnet structures. By adjusting the set screws 33 in alignment with the Y axis, the pole faces can be til-ted slightly to correct their parallelism in the Y direction. Likewise, adjustment of the set screws 33 in alignment with the X axis permits adjusting the parallelism of the pole faces in the X direction. These set screws 33 when properly adjusted reduce the X and Y gradients to less than 10 milligauss per inch. The remaining gradients can then be cancelled by electric shims to produce a field having X, Y and Z linear gradients less than 1 milligauss per inch. Such a highly homogeneous magnetic field in the gap 13 is suitable for high resolution gyromagnetic resonance spectroscopy.

In a typical example of a magnet 1 of the present invention, the egg-shaped yoke 4 is about 14" in diameter at its widest point, about 19" long and the yoke shell is 4 about 0.75" thick. The magnets 8, 9 and 11 are each about 6" in diameter and about 2.5" thick.

Since many changes could be made in the above construction and many apparently widely different embodiments of this invention could be made without departing from the scope thereof, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. In a permanent magnet apparatus, means forming first and second coaxially disposed permanent magnet structures, said permanent magnet structures being axially spaced apart and axially magnetized to provide a pair of axially spaced near poles of opposite polarity defining a magnetic gap therebetween and a pair of remote poles of opposite polarity, means forming a magnetically permeable yoke structure enclosing said permanent magnets and interconnecting said pair of remote poles of opposite polarity to provide a low reluctance flux return path between said magnet structures and to shield the magnet gap from external magnetic fields, the improvement wherein, said enclosing magnetic yoke structure includes first and second axially separable sections which are joined together at a mating circumferentially directed joint, said first permanent magnet being fixedly secured to said first yoke section and said second permanent magnet being fixedly secured to said second yoke section, and means disposed at said mating circumferentially directed joint for adjusting the relative alignment of said first and second magnet structures to adjust the homogeneity of the magnetic field in the gap of the magnet apparatus.

2. The apparatus of claim 1 wherein said mating joint includes means defining first and second radially spaced axially extending aligning surfaces, said first aligning surface being formed on said first section of yoke and said second aligning surface being formed on said second section of yoke, and means for adjusting the radial spacing between said first and second aligning surfaces at a plurality of points about the circumference of said mating joint, whereby the axial alignment of said permanent magnet structures can be adjusted to reduce certain residual magnetic field gradients in the gap of the magnet apparatus.

3. The apparatus of claim 1 where in said mating joint includes means defining first and second axially spaced radially extending aligning surfaces, said first aligning surface being formed on said first section of yoke and said second aligning surface being formed on said second section of yoke, and means for adjusting the axial spacing between said first and second aligning surfaces at a plurality of points about the circumference of said mating joint, whereby the axes of said first and second magnet structures may be tilted with respect to each other to reduce certain residual magnetic field gradients in the gap of the magnet apparatus.

4. The apparatus of claim 1 wherein at least one of said permanent magnet structures is aflixed at its remote pole to said enclosing magnetic yoke structure, and means forming a plurality of jack screws threaded into said yoke structure at a plurality of points off the axis of said magnet structure for exerting adjustable axially directed forces on the end of said magnet structure at a number of points off the axis thereof, whereby the axes of said first and second magnet structures may be tilted relative to each other to reduce certain residual magnetic field gradients in the gap of the magnet apparatus.

5. The apparatus of claim 2 wherein said means for adjusting the radial spacing between the aligning surfaces includes a set of adjusting screws positioned at intervals around the circumference of said mating joint.

6. The apparatus of claim 3 wherein said means for adjusting the axial spacing between the aligning surfaces includes at least one thin strip of shim stock extending only 3,437,963 5 6 partially around the circumference of said mating joint 3,250,961 5/1966 Parker 335-306 XR and disposed between said aligning surfaces.

GEORGE HARRIS, Primary Examiner. References Cited U.S. Cl. X.R. UNITED STATES PATENTS 5 335 3 1 3,018,422 1/1962 Seaton 335*298 3,187,237 6/1965 Craig et a1. 335301 

